The instant invention relates to ceramic bodies or structures having compositions within the Li2Oxe2x80x94Al2O3xe2x80x94SiO2 (lithium aluminosilicate) system. Specifically, the present invention relates to lithium aluminosilicate ceramics having a low coefficient of thermal expansion (CTE), high heat capacity, high refractoriness, and high thermal shock resistance.
In the industry cordierite (2MgOxe2x80x942Al2O3xe2x80x945SiO2) has been the cost-effective material of choice for high temperature filtering applications, such as flow-through and wall-flow filters, due to its combination of good thermal shock resistance, filtration efficiency, and durability under most operating conditions.
However, under certain circumstances cordierite filters are susceptible to damage and have even catastrophically failed.
A need therefore exists for a ceramic suitable for high temperature filtering applications without the shortfalls of cordierite.
The present invention provides such a ceramic and a method of fabricating the same.
The instant invention is founded upon the discovery of a predominately two-phase ceramic within the Li2Oxe2x80x94Al2O3xe2x80x94SiO2 system which has high refractoriness, high resistance to thermal shock, and high heat capacity properties which make the inventive ceramic extremely desirable in high temperature applications, such as filters for diesel exhaust engines.
Specifically the invention is a ceramic article which consists essentially, by weight on the oxide basis, of 10-25% SiO2, 65-85% Al2O3, and 2-12% Li2O and is composed of a first phase having anisotropic thermal expansion behavior (widely differing expansions along the crystallographic axes) with an average coefficient of thermal expansion from room temperature to 1000xc2x0 C. of xe2x88x925xc3x9710xe2x88x927/xc2x0 C. and being less than 50% by weight of the ceramic article, and a second phase having a higher melting point than the melting point of the first phase. The melting point of the second phase is preferably at least 1800xc2x0 C.
The inventive ceramic structures have 32 to 50 weight % of beta-eucryptite (LiAlSiO4) as a first phase having a melting point Tm1, and 50 to 68 weight % of a second phase having a positive component in thermal expansion which is higher than the component in thermal expansion of the first phase and a melting point Tm2, wherein Tm2 greater than Tm1. The second phase is selected from the group consisting of lithium aluminate spinel (LiAl5O8), lithium aluminate (LiAlO2), corundum (Al2O3), and combinations thereof.
The inventive ceramic structures exhibit a coefficient of thermal expansion (CTE) from room temperature to 800xc2x0 C. of xe2x88x9230xc3x9710xe2x88x927/xc2x0 C. to +30xc3x9710xe2x88x927/xc2x0 C., preferably xe2x88x9220xc3x9710xe2x88x927/xc2x0 C. to +10xc3x9710xe2x88x927/xc2x0 C.; a permeability of at least 0.5xc3x9710xe2x88x9212 m2, preferably 1.0xc3x9710xe2x88x9212 m2 to 5.0xc3x9710xe2x88x9212 m2; a total porosity of 35-65%, preferably 45-55%; a median pore size of 8-25 micrometers, preferably 15-20 micrometers; and, a high refractoriness at temperatures of between 1550xc2x0 C. to 1650xc2x0 C.
The inventive ceramic structures are suitable in high temperature applications such as filters for diesel exhaust and automotive catalytic converters. In particular the inventive structure is especially suitable as a honeycomb diesel particular filter having an inlet end and an outlet end and a multiplicity of cells extending from the inlet end to the outlet end, the cells having porous walls, wherein part of the total number of cells at the inlet end are plugged along a portion of their lengths, and the remaining part of cells that are open at the inlet end are plugged at the outlet end along a portion of their lengths, so that an engine exhaust stream passing through the cells of the honeycomb from the inlet end to the outlet end flows into the open cells, through the cell walls, and out of the structure through the open cells at the outlet end.
The invention is also a method of making the ceramic article. A mixture of lithium carbonate, alumina, clay and/or sand, solvent, optionally binders, lubricants and plasticizers are formed into a plasticized batch, shaped into a green body, optionally dried, and fired at temperatures of 1300xc2x0 C.-1400xc2x0 C. and for a time sufficient to form the product structure.
The invention is a ceramic which is largely biphasic, having as a first phase a low CTE phase and as a second phase a high melting temperature phase (the high temperature phase may include more than a single phase as further described herein below). This unique phase duality renders the inventive structure, highly refractory with a near-zero CTE, thus making it suitable for high temperature applications such as filtering of particulate matter from diesel exhaust streams.
The inventive composition area lies within the Li2Oxe2x80x94Al2O3xe2x80x94SiO2 (LAS) system and consists essentially, by weight on the oxide basis, of about 10-25 SiO2, 65-85 Al2O3, and 2-20 Li2O. The preferred compositional area consists essentially, by weight on the oxide basis, of about 13-20 SiO2, 70-80 Al2O3, and 3.5-10 Li2O. Minor amounts of other refractory oxides, such as ZrO2, Cr2O3, V2O3, and Ta2O5 may optionally be present.
In a preferred embodiment the inventive structure includes 32 to 50% by weight a first phase of beta-eucryptite having a melting point Tm1, and 50 to 68% by weight a second phase having a positive component in thermal expansion which is higher than the component in thermal expansion of the first phase and a melting point Tm2, wherein Tm2 greater than Tm1.
The low CTE phase is beta-eucryptite (LiAlSiO4) which has an average CTE from room temperature to 1000xc2x0 C. of about xe2x88x925xc3x9710xe2x88x927/xc2x0 C., and with a highly anisotropic CTE (i.e., widely differing expansions along the crystallographic axes) at the a-axis of about +80xc3x9710xe2x88x927/xc2x0 C. and at the c-axis of about xe2x88x92170xc3x9710xe2x88x927/xc2x0 C.
However, beta-eucryptite also has a low melting point of about 1410xc2x0 C. Therefore, the amount of beta-eucryptite in the final body is less than about 50 percent by weight, and more preferably between about 32 to 45 weight percent to insure that the effective melting temperature of the final body is not compromised. In other words the majority of the ceramic is composed of the high temperature phase.
The high temperature phase has a melting point higher than that of beta-eucryptite, preferably higher than 1800xc2x0 C. The high temperature phase is selected from the group consisting of lithium aluminate spinel (LiAl5O8), lithium aluminate (LiAlO2), corundum (Al2O3), and combinations thereof. Lithium aluminate spinel has a melting point of about 1960xc2x0 C. Corundum has a melting point of about 2020xc2x0 C. LiAlO2 has a melting point of about 1850xc2x0 C.
All three of these phases have a high CTE. Lithium aluminate spinel has a CTE from room temperature to 1000xc2x0 C. of about 85xc3x9710xe2x88x927/xc2x0 C., while corundum has a CTE from room temperature to 1000xc2x0 C. of 84xc3x9710xe2x88x927/xc2x0 C. It is preferred that the second high temperature phase be lithium aluminate spinel because it is in thermodynamic equilibrium with LiAlSiO4 in the solid state, and also forms a rigid network in combination with liquids near this composition in the partially molten state. Therefore, in an especially preferred embodiment the inventive ceramic comprises about 35 weight % beta-eucryptite and 65 weight % lithium aluminate spinel.
The large CTE mismatch between the beta-eucryptite phase and the high temperature phase promotes microcracking either along grain boundaries between beta-eucryptite crystals or between the beta-eucryptite and the high temperature phases which leads to a CTE over a temperature range from room temperature to 800xc2x0 C. of from xe2x88x9230xc3x9710xe2x88x927/xc2x0 C. to 30xc3x9710xe2x88x927/xc2x0 C., preferably xe2x88x9220xc3x9710xe2x88x927/xc2x0 C. to 10xc3x9710xe2x88x927/xc2x0 C., resulting in excellent thermal shock resistance in the inventive structure. Microcracked bodies tend to bias the CTE towards the most negative CTE component because the opening of microcracks on cooling accommodates the normal positive components.
In addition the inventive structure exhibits high refractoriness at temperatures of 1550xc2x0 C. to 1650xc2x0 C. Refractoriness is a measure of the deformation in the structure when exposed to high temperatures such as above 1500xc2x0 C., for a period of time in duration about 10 hours. The extremely high refractoriness in the inventive structure is believed to be the result of the spinel framework maintaining continuity and the beta-eucryptite rich melt attaching itself to the spinel network.
Another advantage in the inventive structure is high permeability by virtue of high, interconnected porosity and large median pore size. The permeability is at least about 0.5xc3x9710xe2x88x9212 m2, and preferably between about 1.0xc3x971012 m2 to 5xc3x9710xe2x88x9212 m2. Permeability is a measure of how easily a fluid can flow through a porous structure. At a constant temperature and fluid viscosity the permeability depends on the percent open porosity, pore size and how well connected the pores are to one another.
The open porosity is between about 35-65%, by volume, and preferably between about 45-55%, by volume. The median pore size is between about 8-25 micrometers, and preferably between about 15-20 micrometers to maintain good filtration efficiency. Open porosity reported as volume percent and pore size reported as median pore diameter in micrometers are measured by mercury porosimetry.
The invention also relates to a method for fabricating the inventive LAS structure. A mixture is formed from raw materials which include lithium carbonate, an alumina-forming source, a silica-forming source and/or kaolin selected to form a composition which consists essentially, by weight on the oxide basis, of about 10-25 SiO2, 65-85 Al2O3, and 2-20 Li2O, and preferably about 13-20 SiO2, 70-80 Al2O3, and 3.5-10 Li2O. Table 1 reports examples of compositions and the resulting phase assemblages according to the present invention.
Raw materials are blended together with organic constituents that may include plasticizers, lubricants, binders, and solvents. Water may also optionally be added as a solvent. The mixture is shaped into a green body, optionally dried, and then fired at a temperature and for a time sufficient to form the final product structure.
The alumina forming source is a powder which, when heated to a sufficiently high temperature in the absence of other raw materials, yields substantially pure aluminum oxide, and includes alpha-alumina, a transition alumina such as a gamma-alumina or rho-alumina, boehmite, aluminum hydroxide, and mixtures thereof. Alpha-alumina is preferred.
The particle size of alumina-forming source particle size has to be sufficiently large for microcraking to be induced in the final structure, and small enough for good extrusion to occur. The high temperature phase inherits the grain size and morphology from the alumina-forming source. Accordingly the particle size of the alumina-forming source has to be at least 10 micrometers and no greater than 50 micrometers, preferably between about 15 to 25 micrometers; single crystal particles below 10 micrometers there would result in insufficient strain along grain boundaries with adjacent differing CTE to develop microcracking; single crystal particles above 50 micrometers would result in large microcracks which may extend during thermal cycling across webs. The morphology of the alumina source is also important and has to be macrocrystalline with no aggregates of fine crystallites.
The silica-forming source includes, but is not limited to, quartz. Optionally, kaolin (which aids in the extrusion process) may be added, preferably in an amount no greater than 20% by weight.
The inventive structure is particularly suitable for high temperature filtration applications. In particular the inventive structures are particularly suitable for diesel particulate filter applications. For such applications the raw material mixture is preferably shaped by extrusion into a honeycomb multicellular structure, as known in the art.
The resulting shaped green honeycomb bodies are usually dried and heated to a maximum temperature of about 1300-1400xc2x0 C. over a period of about 28 hours, and held at the maximum temperature for about 6-10 hours.
While the construction of the filter can have any shape or geometry suitable for a particular application, it is preferred that it be multicellular structures such as a honeycomb structures. The honeycomb structure has an inlet and outlet end or face, and a multiplicity of cells extending from the inlet end to the outlet end, the cells having porous walls. The inventive filters have cellular densities from about 100 cells/in2 (15.5 cells/cm2) to about 400 cells/in2 (62 cells/cm2).
To obtain a filtering device, a portion of the cells of the honeycomb at the inlet end or face are plugged, as known in the art. The plugging is only at the ends of the cells which is typically to a depth of about 5 to 20 mm, although this can vary. A portion of the cells on the outlet end but not corresponding to those on the inlet end are plugged. Therefore, each cell is plugged only at one end. The preferred arrangement is to have every other cell on a given face plugged as in a checkered pattern.
An advantage of the diesel particulate filters of the present invention have many advantages is a low pressure drop across the length of the filter and low back pressure against the engine comparable to commercially available SiC counterparts. The pressure drop across the filter is a function of the accumulation of the carbonaceous soot on the walls of the diesel particulate filter. As amount of soot accumulated increases, it creates a progressive increase in the resistance to flow of the exhaust gas through the walls of the filter and carbon soot layer. This resistance to flow is manifested as a pressure drop that can be measured across the length of the filter, and results in an increased back pressure against the engine.
Although the preferred application is for diesel particulate filters, it is to be noted that the inventive ceramic is equally suitable as automotive flow-through substrates.